Structure of Molluscan Prismatic Shell Layers

Abstract

Structure of the calcitic megaprisms is studied with SEM in the outer shell layer of two archaeogastropod species: Haliotis rufescens Swainson, from the Pacific at San Francisco, and Haliotis tuberculata L., from the Atlantic, Dinard, France. In H. rufescens where the calcitic megaprisms have a regular size, shape and orientation, the prisms are traversed by horizontal organic intracrystalline sheets. These sheets are regularly spaced (about 1 micron apart) and have a parallel fibrous structure. The occurrence of well defined intracrystalline sheets in the prismatic structures has not been reported previously. In H. tuberculata where the calcitic megaprisms occur together with aragonitic granules, and are precipitated at irregular intervals, the prisms have a highly varying size, shape and orientation. The intracrystalline organic matrix seems to form also here horizontal sheets but these are fragile and composed of very thin fibres with reticulate arrangement. Consequently, there exists a close correlation between the organization of the calcitic megaprisms and the intracrystalline organic matrix.

Extremely thin, horizontal, organic sheets are described in the semi-prismatic shell layer in the cephalopod Nautilus pompilius. The spacing of these sheets is similar to that of the interlamellar organic sheets in the adjacent nacreous layer. The following structural changes take place when the semi-prismatic layer is transformed into the nacreous layer: (1) the acicular crystallites acquire a strictly vertical orientation and form nacreous tablets which become twinned, and (2) the simple, horizontal, soluble organic sheets increase in thickness, become three-layered and partially insoluble.